The present invention relates to improvement to the process for the manufacture of alkyl acrylates by direct esterification.
It is necessary in esterification processes, in order to achieve maximum conversion of the alcohol and the acid, to shift the following reaction equilibrium over to the right: EQU R-COOH+R'--OH.rarw..fwdarw.R+COOR'+H.sub.2 O.
With this aim, the most commonly used process consists in distilling off the lightest product, in this case the water, as it is formed. In order to optimize the reaction kinetics, it is important to limit the water present in the reaction medium to the minimum, by extracting it as quickly as possible, in order to avoid any reverse reaction.
In particular, in the field of the synthesis of acrylic esters, which have the reputation of being thermally unstable (polymerization promoted by thermal effect), it is conventional and energetically less expensive to distil off the water in the form of an azeotrope having a lower boiling point.
The azeotrope may be a homo- or a heteroazeo-trope, forming a one- or two-phase system with water respectively. In the case of the distillation of a homoazeotrope, selectivity of the distillation column can only be provided for at the cost of the refluxing of some of this azeotropic mixture to the head of the column. This results in water being returned into the reaction medium, which decreases the kinetics of the esterification reaction.
In contrast, if the mixture is a heteroazeotrope, this may be separated into an upper organic phase and a lower aqueous phase in a decanter located at the head of the distillation column, and the refluxing necessary to ensure good selectivity in the column my be obtained by recycling the single organic phase. In this case, the water is usually withdrawn continuously at the bottom of the decanter.
The substance used to entrain the water of the reaction azeotropically is generally an immiscible solvent which forms an azeotrope with water, as is described in American patents U.S. Pat. No. 5,093,520 and U.S. Pat. No. 2,917,538; however, this process has the drawback of adding a new substance to the reaction mixture, which will have to be separated out afterwards and possibly purified before recycling upstream in the process.
The use of the esterifying alcohol as azeotroping solvent, particularly if it is immiscible with water, is also described, thereby avoiding the use of a new substance in the process.
In the case of the manufacture of acrylic esters of alcohols having a number of carbon atoms greater than or equal to 5, such as 2-ethylhexyl acrylate, described in American patent US Pat. No. 4,280,009, this process cannot be used in a simple manner alone. The heteroazeotrope formed by the ester-alcohol-water mixture has a high boiling point which is relatively close to that of acrylic acid. As a result, some of the acrylic acid distils at the same time as the water and the entraining alcohol. In general, for the esterification of alcohols having a boiling point higher than that of acrylic acid, the lightest compound (acrylic acid) is entrained in the vapors, especially when, at the end of the reaction, the amount of water to be extracted from the reaction medium, which is required to form the light azeotropic mixture, becomes very low.
The entrainment of acrylic acid thus becomes particularly considerable when quantitative conversion of the reactants is intended.
After separation, a large proportion of this acrylic acid is found in the aqueous phase, which gives rise to a loss of product and an increase in the organic pollution (COD) of the water discharged from the plant.
With reference to FIG. 1 of the attached diagram, it may be seen that, in an esterification performed according to the usual technique, the vapors condensed at the head of the distillation column (DC) during the reaction (heteroazeotropic mixture M) are collected in a decanter (D) where the mixture separates out into two phases. The upper organic phase (O) is returned by overflow (R) (natural refluxing) to the head of the column (DC). The aqueous phase (A) is, itself, withdrawn continuously via a valve (V) which regulates a constant interphase level in the decanter (D).
When this process is applied to the synthesis of heavy acrylates, the content of acrylic acid in the aqueous phase recovered after reaction is very large.
In order to decrease the entrainment of acrylic acid in the azeotrope distilled off during the reaction, the amount of backstreaming within the column may be increased by introduction of some of the esterifying alcohol to the head of this column, as is described in American patents U.S. Pat. No. 4,833,267 and U.S. Pat. No. 4,280,009, in Romanian patent RO-70,951 and in Japanese patent application JP-A-58,192,851.
This method requires the addition, via the head of the column, of a large proportion of alcohol relative to the total amount of alcohol required for the reaction, and the technique is insufficient for avoiding the distillation of acrylic acid in the case of a synthesis of 2-ethylhexyl acrylate, particularly when total conversion of the reactants is intended.
Moreover, American patent U.S. Pat. No. 4,076,950 describes an esterification process aimed at distilling off, during the reaction, an azeotropic mixture which is free of acrylic acid. In order to do this, the organic phase which has been freed beforehand of the starting alcohol by extraction with water, and which then essentially contains anhydrous ester, is refluxed to the column head. This procedure cannot be adapted in the case of esterification of heavy alcohols (C.sub.n alcohols where n.gtoreq.5), on account of the very low solubilities of the alcohols in water.
The problem which the Applicant Company has sought to solve, in the synthesis of acrylates manufactured from alcohols having a number of carbon atoms greater than or equal to 5, by direct esterification, performed continuously or discontinuously in the absence of external azeotroping solvent, is that of minimizing the content of acrylic acid in the waste water discharged from the plant, in order to reduce the loss of this monomer and the organic pollution (COD) which it entails.
To this end, provision is made, in accordance with the present invention, to return the acrylic acid entrained during the distillation of the heteroazeotropic mixture to the head of the distillation column mounted over the reactor, after extraction of this acid contained in the reaction water with an esterifying-alcohol-rich mixture.
The subject of the present invention is thus a process for the manufacture of an acrylic acid ester by reaction of the said acid and an alcohol chosen from primary and secondary aliphatic monoalcohols having a number of carbon atoms greater than or equal to 5, in the presence of an acid as esterification catalyst and of a polymerization inhibitor, the alcohol being capable of forming a heteroazeotrope with water, the water formed by the esterification reaction being entrained by distillation in a column in the form of a heteroazeotropic mixture with the alcohol, which mixture, containing an amount of unreacted acrylic acid, is then subjected, after condensation, to a separation in order to give an upper organic phase which is recycled to the head of the distillation column and a lower aqueous phase which is withdrawn, characterized in that an extraction of the acrylic acid contained in the condensed heteroazeotropic mixture intended for phase separation is carried out with an esterifying-alcohol-rich mixture, and in that the acrylic acid thus extracted is returned to the head of the distillation column, as a constituent of the recycled organic phase, with an increase in the level of backstreaming in the distillation column relative to the natural refluxing.
Various means allow this increase in the level of reflux in the distillation column, these means possibly depending in particular on whether continuous or discontinuous conditions are used.